1. Technical Field
The present invention relates to a method for controlling a servomotor, and more particularly, to a drive control method for a servomotor, in which a high output can be fetched from the servomotor by means of a motor-current control unit of a simple construction.
2. Description of the Related Art
The recent mainstream of servomotor drive control is PWM control using semiconductor switching elements, such as power transistors, thyristors, etc. Also, there is a demand that a high output should be obtained from servomotors. In order to obtain a high output from a servomotor, a large current must be supplied to the semiconductor elements. It is difficult, however, to obtain semiconductor elements which permit a large current to be applied thereto. Even if a large current is allowed to flow through a semiconductor element, moreover, the semiconductor element has the property of generating heat in its active region, and hardly in its cut-off region and saturated region. Therefore, the calorific value of the semiconductor element increases in proportion to the chopping frequency, so that the chopping frequency must be lowered. If the chopping frequency is lowered, however, motor control inevitably becomes difficult. Thereupon, a tentative arrangement is provided such that each of the switching elements of a PWM control section is composed of a plurality of semiconductor elements, and the driving current of the servomotor is increased by reducing the currents flowing through the individual semiconductor elements. In such a case, however, current concentration in the semiconductor elements (or large current flow only in some of the semiconductor elements) is caused, so that the driving current cannot be increased in proportion to the number of parallel-connected semiconductor elements.
Conventionally, therefore, a high output is obtained by operating a plurality of motors in a parallel manner. In this case, however, it is necessary to mechanically connect the individual motors with one another and with an output shaft. This results in an increase in cost.